This river has a remarkably steady discharge in a region where most rainwater infiltrates to the groundwater table without contributing much surface runoff. You can read more about the hydrology of the Dismal River here: http://pubs.usgs.gov/circ/circ1173/circ1173c/pdf-c/chapter06c.pdf
From the ground it's not easy to sense that the hills of this area are actually thinly vegetated sand dunes, though this is readily apparent in the Google Earth satellite view of the area. A small tweak of the climate could turn this area into a sand sea like those found in the Sahara Desert on Earth or the region near Mos Eisley on Tattoine. In fact, just such a dune sea covered much of Nebraska within the very recent geologic past.